The sphygmomanometric class of automated blood pressure monitors employs an inflatable cuff to exert controlled counter-pressure on the vasculature of a patient. One large class of such monitors, exemplified by that described in U.S. Pat. Nos. 4,349,034 and 4,360,029, both to Maynard Ramsey, III and commonly assigned herewith, employs the oscillometric methodology.
In accordance with the Ramsey patents, an inflatable cuff is suitably located on the limb of a patient and is pumped to a predetermined pressure above the systolic pressure. Then, the cuff pressure is reduced in predetermined decrements, and at each level, pressure fluctuations are monitored. The resultant signals typically consist of a DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations (referred to herein as "oscillation complexes" or just simply "oscillations").
After suitable filtering to reject the DC component and to provide amplification, peak pulse amplitudes (PPA) above a given base-line are measured and stored. As the decrementing continues, the peak amplitudes will normally increase from a lower level to a relative maximum, and thereafter decrease. These amplitudes thus form an oscillometric blood pressure envelope for the patient. The lowest cuff pressure at which the oscillations have a maximum value has been found to be representative of the mean arterial pressure ("MAP"). Systolic and diastolic pressures can be derived either as predetermined fractions of MAP, or by more sophisticated methods of direct processing of the oscillation complexes.
The step deflation technique as set forth in the Ramsey patents is the commercial standard of operation. A large percentage of clinically acceptable automated blood pressure monitors utilize the step deflation rationale. When in use, the blood pressure cuff is placed on the patient and the operator sets a time interval, typically from 1 to 90 minutes, at which blood pressure measurements are to be made.
The noninvasive blood pressure ("NIBP") monitor automatically starts a blood pressure determination at the end of the set time interval. Typically, the user selects a short interval if the patient is unstable because blood pressure may change to dangerous levels in a short time and selects a longer interval as the patient becomes more stable. The reason a short interval is not used in all cases is that the probability of trauma to the limb from the cuff inflation increases as the determination frequency increases.
U.S. Pat. No. 5,606,977, to Maynard Ramsey, III and commonly assigned herewith, employs the oscillometric methodology. This patent describes an automated sphygmomanometer which automatically determines when a blood pressure determination needs to be made and automatically determines whether the patient's blood pressure has changed significantly since the last determination so that a new blood pressure determination may be instigated immediately. This patent also discloses a technique for monitoring the status of the patient's blood pressure between determinations so that a change in status between blood pressure determinations will not go undetected. In addition, this invention discloses a technique for monitoring the status of the patient's blood pressure so that full blood pressure determinations need not be taken as frequently, thereby minimizing the possibility of trauma to the patient.
Automated NIBP techniques are normally utilized when constant monitoring of the patient's blood pressure is desired such as during surgeries, less serious medical procedures, or in ambulances on the way to a hospital. Thus, the accuracy of the PPA readings, the resulting MAP, and systolic and diastolic calculations are critical to the success of the procedure and the health of the patient.
It is, therefore, imperative that the oscillation complexes are rue representations of the patient's blood pressure. Patient movement, however, creates noise which interferes with accurate measurement of oscillation complexes using an inflatable blood pressure cuff. In particular, noise is caused when movement of the patient's arm causes the inflatable blood pressure cuff to contact an external object, thereby compressing the cuff between the object and the patient's arm. Such noise is particularly problematic in an ambulance or in an emergency room environment. This contact and resulting compression creates a noise pressure signal which interferes with accurate measurement and monitoring of the oscillation complexes. The noise will sometimes occur at the same time as an oscillation complex, and therefore, may be very difficult to discount as noise. Depending on the severity of contact, such noise can be mistaken for an oscillation complex.
Of course, devices are known that provide support to a patient's limb. U.S. Pat. No. 4,315,504, entitled "Elbow Suspension Device," describes such a device for bed and ambulatory patients which provides support above and below the elbow, thus protecting the elbow. U.S. Pat. No. 4,922,929, entitled "Padded Elbow Brace," describes such a device for protecting limb joints with cushioning material. U.S. Pat. No. 4,790,325, entitled "Automatic Arterial Blood Pressure Recorder," describes such a device which utilizes an arm rest built into the arm rest of a chair. The disclosed arm rest provides an ergonomic fit for the patient's arm and serves to position the arm so that automatic blood pressure monitoring can be performed. However, an obvious problematic feature of this device is that the arm rest is in direct contact with the inflatable blood pressure cuff, leading to interference from noise.
At a minimum, noise caused when the inflatable cuff contacts external objects interferes with accurate blood pressure measurement. It is, accordingly, an object of the present invention to reduce the amount of noise that interferes with accurate blood pressure measurement and monitoring with automated noninvasive blood pressure devices by providing a device which minimizes cuff contact with external objects during such measurement.